Acoustic lens

ABSTRACT

An acoustical lens doublet of the type particularly useful for focusing sonic energy on the energy converter array of an electroacoustical transducer having image resolution powers that are substantially unaffected by varying temperatures and pressures ambient thereto is disclosed. Included therein is a rearward shell containing a low density polyethylene acoustically clear forward wall-hemispherical lens combination. A forward shell is telescopically mounted on the front end of said rearward shell, with the forward end of said forward shell having a thin low density polyethylene acoustically clear window located therein. An interface fluid, preferably consisting of a predetermined quantity of m-chloroanoline (ClC6H4NH2), is located under a predetermined pressure within a chamber effected between the forward ends of said rearward and forward shells, as a result of the telescopic relationship thereof.

United States Patent Rolle Dec. 4, 1973 ACOUSTIC LENS Primary ExaminerBenjamin A. Borehelt Assistant Examiner.l. V. Doramus 75 1 t A1betL.Rl1 P Ct Fl. nven or r o anama I a Attorney-Richard S. Sciascia et al. [73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC. [57] ABSTRACT 22 Filed: p 1972 An acoustical lens doublet of the type particularly useful for focusing some energy on the energy converter Appl. No.: 241,602

array of an electroacoustical transducer having image resolution powers that are substantially unaffected by varying temperatures and pressures ambient thereto is disclosed. Included therein is a rearward shell containing a low density polyethylene acoustically clear forward wall-hemispherical lens combination. A forward shell is telescopically mounted on the front end of said rearward shell, with the forward end of said forward shell having a thin low density polyethylene acoustically clear window located therein. An interface fluid, preferably consisting of a predetermined quantity of m-chloroanoline (ClC H Nl-1 is located under a predetermined pressure within a chamber effected between the forward ends of said rearward and forward shells, as a result of the telescopic relationship thereof.

15 Claims, 7 Drawing Figures PATENTEDUEC M975 3.776361 AIvIBIENT TO (Fl) 62 I I (C ENVIRONMENT 63 Z 64 83 FIG. 6 75 CARRIER VEHICLE [/77 I 78 76 I READOUT Q PRESSURE I PRESSURE I SUPP CONTROLLER SENSOR I LY J 8| WATER I 74 I TRANSDUCER [/72 T ,82 I

I 73 I ENERGY I 79 I I soNAR coNvERTER LENS H ZI TARGET I ARRAY I I FIG. 7

z I 7 PRIOR ART TRANSDUCER O I ENs IN sEA 0 WATER m u- LL] m I s A LA.

O I X SUBJECT INvENTIoN J III TRANSDUCER LENS 3 IN sEA WATER |.5

l l l I O 30 4O 5O TEMPERATURE C ACOUSTIC LENS STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore.

FIELD OF THE INVENTION The present invention relates, in general, to lens and lens systems, and, in particular, it is an athermal acoustic refractor that may be advantageously combined with an array of electroacoustical energy converter devices for the focusing of acoustical images thereon or therefrom. In even greater particularity, but not -by way of limitation, it is an improved, reversible, temperature and salinity compensated, compound, ultrasonic, electroacoustical transducer lens that is eminently suitable for being incorporated in both active and passive sonar systems, in sonic echo-searchranging systems, and in any other devices or systems requiring the accurate focusing of acoustical energy under ambient environmental conditions of varying temperature and/or salinity.

DESCRIPTION OF THE PRIOR ART Heretofore, numerous electroacoustical transducers have incorporated various and sundry acoustic lenses, in order to effect more accurate focusing of sonic energy therein. For many practical purposes, such lenses have been quite satisfactory; however, in the event that environmental medium ambient thereto varies in temperature, the index of refraction thereof is changed, thereby, in turn, effecting a change in the focal lengths thereof. Of course, such changes in focal length ordinarily change the focus of such prior art transducers enough to cause unwanted acoustical image aberrations to occur therein.

In the past, the adverse effects of changing temperatures on the index of refraction of an acoustical lens was compensated by adjusting the distance between the lens and the electroacoustic energy converter elements, so that the refracted acoustic images would be properly in focus therein. Unfortunately, if such lens is employed to form several acoustic beams over a significantly large field of view, a change in focal length also necessitates a change in the curvature of the transducers energy converter array at the image plane. Because it is exceedingly difficult to accurately control lens focal lengths and image plane curvatures, using such procedures is usually impractical.

Another means for compensating an acoustical lens for changing temperature is to change the composition of the lens itself, so as to correct the index of refraction thereof and, thus, adjust the focus thereof. Again, for most practical purposes, so doing is too difficult and time consuming to be effective, at least within most practical time limits.

In addition, acoustic lenses have been made of materials that inherently change the index of refraction thereof in compensating proportionto the temperature change to which they are exposed, and again, for many practical purposes, such lenses have proven to be quite satisfactory. But, to date, for some purposes-say, for example, during deep sea sonar operationsthe temperature compensation effected thereby is not sufficient or is not sufficiently proportional to the actual temperature gradients encountered to be as accurate as desired. Hence, in many instances, they leave a great deal to be desired when actually employed in various and sundry operational circumstances.

SUMMARY OF THE INVENTION Perhaps the perfect method and means for compensating for temperature changes in acoustical lenses for all operational circumstances may not be found for quite some time, if ever; nevertheless, for many practical purposes, the temperature compensating ultrasonic lens of the subject invention constitutes a vast improvement over most of the prior art, in that the index of refraction thereof varies less per degree change in temperature for a very important and useful temperature range, due to the new and unique inherent temperature compensation characteristics existing therein, as will be shown more vividly subsequently. As a result, the versatility of electroacoustical transducers incorporating such lenses has become considerably greater than heretofore obtainable from those of the prior art.

Briefly, the instant invention consists of a pair of cylindrical shells that are constructed of acoustically clear material which are assembled together in such manner that the forward one herein defined as being an interface shell is telescopically mounted on and I partially around the rearward one herein defined as being a lens shell. When so assembled, a forward looking chamber is formed between the interface and lens shells that is filled or partially filled with an organic fluid herein defined as being the interface fluid the change of acoustical index of refraction of which is opposite that of the environmental medium ambient thereto for any given identical temperature change .in both thereof. As will be described in greater detail subsequently, said interface fluid may be supplied under pressure from an external source or a predetermined quantity thereof may be confined therein, depending upon the operational circumstances such as, for example, the considerable ambient pressures encountered at great water depths in an ocean.

The aforesaid lens shell of the unique interface shell lens shell assembly constituting this invention is con-. structed to have such a convex configuration as to cause it to inherently refract acoustical energy in such manner as to effect the focusing thereof at a focal point or spot located at a given radius or focal length (fl) therefrom in the rearward direction. In contact with the rearward face, and optionally, but preferably, filling a predetermined space behind it, is another organic fluid herein defined as being the lens fluid which may or may not have an acoustical index of refraction that varies inversely' with that of the aforesaid environmental medium for any given identical temperature in both thereof.

Because the electroacoustic energy converters of the transducer are also either actually or effectively lo-- cated in the aforementioned lens fluid, and because they are ordinarily located alongthe focal length or radius of the lens shell, the acoustical energy received by the lens shell is focused thereon with a relatively high degree of accuracy, regardless of the fact that the temperature ambient thereto may be changing rapidly. Hence, the entire transducer has a responsiveness, a resolution, and an image fidelity hitherto unattainable by any known prior art.

It is, therefore, an object of this invention to provide an improved acoustical lens.

Another object of this invention is to provide an acoustical lens having improved automatic temperature compensation characteristics.

Still another object of this invention is to provide a new and unique system for acquiring and imageprocessing the acoustical energy emanating from an underwater target object.

A further object of this invention is to provide an acoustical lens having an acoustical index of refraction that varies only a small amount with temperature.

Another object of this invention is to provide an acoustical lens which is not put out of focus as a result of being transferred from a medium having a particular sound velocity to another medium having a different sound velocity, such as, for example, when going from fresh water into sea water and vice versa.

Another object of this invention is to provide an improved method and means for focusing acoustical energy.

Still another object of this invention is to provide an improved electroacoustical transducer sonic lens that requires no focus adjustment over a relatively large temperature range.

Still another object of this invention is to provide an electroacoustical transducer lens with improved power transfer characteristics and reduced internal reflections.

A further object of this invention is to provide a more versatile method and means for focusing sonic and ultrasonic energy.

Another object of this invention is to provide an acoustic refractor that is substantially uneffected by changes in the temperature thereof and its ambient environment.

Another object of this invention is to provide a compound lens having fast sound velocity propagation components.

Another object of this invention is to provide an improved sonic and ultrasonic compound lens which exhibits small changes in focus over relatively wide temperature ranges.

Another object of this invention is to provide improved method and means of forming high fidelity acoustical images when incorporated in a compatible sonar system.

Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of the cylindrical lens shell of the subject acoustical lens invention;

FIG. 2 is a cross-sectional view of another species of a cylindrical lens shell that may be incorporated in the instant acoustical lens invention;

FIG. 3 is a cross-sectional view of the cylindrical intcrface shell of the subject acoustical lens invention;

FIG. 4 is a cross-sectional view of a preferred embodiment of the cylindrical compound lens constituting the invention, which includes the lens shell of FIG. I and the interface shell of FIG. 3 combined as a unitary assembly and which, by implication, portrays how the lens shell of FIG. 2 may be incorporated therein by the artisan instead of the lens shell of FIG. 2, if so desired;

FIG. 5 is a schematic representation of the invention which will be used subsequently in disclosing the theory of the operation thereof;

FIG. 6 is a block diagram of a sonar system which may incorporate the instant invention to an advantage;v

and

FIG. 7 is a graphical representation of typical index of refraction versus temperature curves for electroacoustical transducers operating within approximately a forty-five degree Centigrade temperature range, both with and without the incorporation of an M- chloroaniline (ClC l-l NH interface fluid therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown an acoustical lens shell 11 having a cylindrical section 12, with an open rearward end 13 and a closed forward end wall 14. In the disclosed embodiment of forward end wall. 14, a flat wall 15 is included, and in the center of said flat wall is an aperture 16 to which a hemispherical lens 17 is integrally connected at substantially the maximum diameter thereof. At this time, it would perhaps be noteworthy that said flat wall 15 and said hemispherical lens 17 are constructed of acoustically clear material, and that the thickness and diametrical dimensions thereof should be such as would optimize the acoustical clearness thereof, respectively. For example, but not by way of limitation, wall 15 and lens 17 may be con structed of low density polyethylene having a thickness of the order of 0.070 inches, with the inside radius of lens 17 being of the order of 0.75 inches. Of course, although the foregoing polyethylene rnaterial and dimensions are quite satisfactory, it should be understood that others may be used if operational circumstances so warrant. Moreover, the entire shell constituting lens shell 1 1, including forward flat wall 15 and hemispherical lens 17, may be so designed as to be of other plastics, rubber, metal or any other suitable acoustically clear materials. Of course, under such circumstances, the dimensions therefor would have to be selected, respectively, if optimum operation is to be obtained therefrom for any predetermined purpose. Obviously, the selection of the appropriate materials and dimensions for lens shell 1 1 would be well within the purview of one skilled in the art having the benefit of the teachings presented herein.

At the rearward end of cylindrical lens shell 11, and preferably integrally connected to cylindrical section 12 thereof, is a circular flange 18 having a plurality of holes 19 extending therethrough and disposed uni formly around it adjacent to the periphery thereof. Said flange l8 and holes 19 are adapted to be respectively complementary and in line with those components (not shown) of a transducer case or housing to which flange 18 is to be bolted, and, in addition, with other lens elements to be described subsequently.

FIG. 2 depicts another preferred embodiment of an acoustical lens shell 21 that may be substituted for the aforementioned lens shell 11 in the completed lens assembly, which, likewise, will be discussed below. Shown therein is a cylindrical section 22, with a flat, acoustically clear wall 23 located at the rearward end thereof for the closure thereof and a closed forward end 24 effected by a flat wall 25 effectively having an aperture 26 located at the center thereof. Integrally connected to wall 25 at the periphery of aperture 26 is a hemispherical lens 27. Like the aforementioned acoustical lens shell 11, acoustical lens shell 21 is made of any acoustically clear material and have whatever dimensions are suitable therefor, in order to optimize the operation thereof for any given purpose within any given environmental medium. Hence, the previously mentioned materials and dimensions are quite satisfactory for the lens shell embodiment being discussed at this time.

At the rearward end of cylindrical lens shell 21 and preferably integrally connected to cylindrical section 22 thereof is a circular flange 28 having a plurality of holes 19 extending therethrough and disposed uniformly around it adjacent to the periphery thereof. Said flange 28 and holes 29, like the aforesaid flange l8 and holes 19 of lens embodiment 11, are so designed as to be respectively compatible with those components (not shown) of a transducer case or housing to which flange 18 is intended to be bolted, and again, in addition, with other lens elements to be described subsequently.

The foregoing structure of FIG. 2 obviously forms a closed chamber 31 which is filled with an organic sound conducting fluid 32, the sound velocityof which is inversely proportional to the temperature thereof and, thus, decreases with an increase in temperature and vice versa. Although numerous conventional organic fluids exist which have suitable characteristics for acoustic lens purposes, it has been determined that dibromethylene (CI-I BR carbon tetrachloride (CCl chloroform (CHCI methylene iodide (CI-I 1 and Halocarbon Oil, Ser. No. 208, manufactured by the Halocarbon Products Corporation of Hackensack, N.J., are satisfactory for such purpose, since the velocity of sound passing therethrough is such as facilitates the focusing thereof on an energy converter array, as will be discussed more fully below.

For the purpose of filling chamber 31 with fluid 32, a threaded hole 33 is located through the wall of cylinder section 22. And for the purpose of containing fluid 32 in chamber 31, a threaded plug 34 is screwed in threaded hole 33. Because, as previously suggested, the interface shell will be telescopically mounted on and around lens shell 21, plug 34 should be so designed as to make the outer end thereof at least flush with and preferably recessed from the outer diameter cylinder section 22, when it is inserted in hole 33.

Referring now to FIG. 3, there is shown a preferred embodiment of an interface shell 35 which may be telescopically mounted on and around either lens shells 11 or 21 of FIG. 1 or FIG. 2, in order to effect the new combination of elements constituting this invention. Disclosed therein is a cylindrical section 36 having an open rearward end 37 and a closed forward end 38. The closure at the forward end is, in fact, effected by a wall having a thick section 41 near the periphery thereof and a relatively thin circular acoustical window section 42 at the center portion thereof. For example, said thick wall section 41 may be of the order of 0.125 inches thick, and said thin acoustical window 42 may be of the order of 0.025 inches thick. Integrally attached to the rearward end of cylindrical section 36 is a flange 43 having a plurality of holes 44 extending therethrough and uniformly spaced therearound.

As will be more fully disclosed below, the inside diameter of interface shell 35 should be such as would cause it to fit snugly around the outer diameter of either of the lens shells 11 and 21 of FIGS. 1 and 2, re-

spectively, and thereby'effect a fluid tight seal therebetween. Moreover, flange 43 and holes 44 should be so designed as to have a complementary 'fit and be in alignment with flanges 18 or 28 and holes 19 or 29, re-

spectively, when telescopically mounted around either of lens shells 11 or 21.

Located around the inside diameter of cylindrical section 36 of interface lens 35 is a groove 45 adapted for having a resilient O-ring (not shown in FIG. 3) inserted therein that will, as will be subsequently disclosed, provide further liquid or fluid seal between the inner surface of interface shell 35 and the outer surface of the lens shell telescopically mounted therein.

As in the shells of FIGS. 1 and 2, the walls of interface shell 35 is made of an acoustically clear material, such as, for instance, a low density polyethylene, rubber, metal, or the like. Again, the material selected therefor should be compatible with the materialselected for the co-acting lens shell combined therewith.

Located in the wall of the cylindrical section 36 of shell 35 is a threaded hole 46 adapted to be used for.

47 is inserted in threaded hole 46 to prevent leakage of the organic fluid or fluid therefrom.

As an alternate possibility, hole 46 of interface shell 35 may have a pipe nipple screwed therein, which would facilitate the connection of other suitable piping thereto, in the event the subject lens were incorporated, for example, in a system such as that illustrated in FIG. 6.

FIG. 4 consists of an acoustical lens assembly 51 of the aforementioned lens shell 11 in combination with the interface shell 35. In order to'keep this disclosure as simple as possible, only an assembly incorporating lens shell 11 has been shown; however, it should be understood, as previously suggested, that lens shell 21 may be substituted therefor, in the event that a unitary, self-contained, compound, acoustical lens is desired. Obviously, so doing would be well within the ability of the artisan constructing and using it, inasmuch as the teachings thereof are explicitly included herein. Furthermore, insofar, as it is possible, in order to correlate the various and sundry elements of the acoustical lens of FIG. 4 with identical parts of the shells depicted in FIGS. 1 and 3, like reference numerals have been employed for like parts. Moreover, because all of the elements of acoustical lens 51 would readily be recognized.

by the artisan as being the same as those shown in FIGS. 1 and 3, respectively, it will only be discussed briefly from a structural standpoint, with only those additional structural explanations made which are deemed necessary for the teaching thereof as an assem- Y bled unit. Hence, lens shell 11 is shown as being telescopically disposed within interface shell 35, with the rearward surface of flange 43 of interface lens 35 being in complementary abutment with the forward surface of flange 18 of lens shell 11. Of course, the respective pluralities of the aforesaid holes 44 and 19 are in align: ment, too.

The particular acoustical lens embodiment of FIG. 4 intended to be attached to the housing of the overall electroacoustical transducer. Because said transducer could have any of many suitable, conventional, geometrical configurations, it is not disclosed in detail herein. Suffice to say, however, that the forward end thereof should be designed in such manner that it would form a complementary connection with the rearward surface of flange 18 of lens shell 11. Thus, a transducer housing 52 likewise contains a flange 53 at the forward end thereof which, in turn, contains a plurality of holes 54 that are located therein in such manner that they are in alignment with holes 19 and 44, respectively, of flanges l8 and 43 of shells 11 and 35.

In order to secure housing 52 to acoustical lens assembly 51, a plurality of threaded bolts 55 are inserted through the aligned ones of holes 54, 19, and 29, and a like plurality of nuts 56 are screwed on the forward ends thereof. Of course, any suitable conventional means, such as, for instance, lock washers or additional nuts, may also be used to insure that the aforesaid nuts 56 will not come loose.

In addition, although not shown, if so desired for any given operational circumstances, appropriately designed gaskets may be disposed between all of the foregoing mating surfaces. One particular sealing means disposed between shells 11 and 35 is an Oring 57 that is located in the aforementioned groove 45 of shell 35 and, thus, is in resilient sealing contact with the outer surface of the cylindrical section of lens shell 11. Hence, the seals between the assembled shells of lens 51 are fluid tight, thereby preventing the entrance of ambients fluids therein, and also preventing the escape of any of the acoustical energy refracting fluids therefrom.

The transducer in which acoustical lens 51 is incorporated is what is commonly known in the art as a liq uid lens transducer. Consequently, housing 52 thereof is filled with an acoustical energy refracting organic liquid, herewith defined as being organic lens fluid 58. Again, as indicated previously with respect to lens fluid 32, lens fluid 58 may be any conventional transducer lens liquid, such as, for example, dibromethylene (CI-I Br carbon tetrachloride (CCl.,), chloroform CHCI methylene iodide (CI-I 1 or Halocarbon oil. Because it also fills the rearward portion of lens shell 11, and because the piezoelectric electroacoustical energy converters (not shown) are also flooded thereby, said lens fluid 58 likewise constitutes a medium which transmits the sonic energy passing through lens wall 17 to said energy converters. And because fluid 58 is a sonic energy refracting liquid, it and hemispherical lens 17 cause the focusing of said sonic energy on the aforesaid electroacoustical energy converters.

Another sonic refracting liquid, interface fluid 59, fills or substantially fills the space between the forward surface of hemispherical membrane lens 17 and the rearward surface of flat wall 42 of interface lens 35. This fluid is preferably the organic compound M- chloroanaline (ClC l-I NH and has an index of refraction that is inversely proportional to that of water. Interface fluid filling may be effected through threaded hole 46, after which plug 47 is inserted. Interface fluid 59, like lens fluid 58, provides a suitable medium for transmitting the operative sonic energy received through forward window 42 to the forward surface of acoustical hemispherical lens 17. Although the aforementioned M-chloroanaline (CIC H NH is the fluid considered to be optimum for this purpose at the present time, other such fluids such as. for instance, nitrobenzoil (C H NO O-kersol (C H 0), and M- nitrotoluol (C H NO would be satisfactory for some operational situations, too.

At this time it would perhaps again be noteworthy that lens shell 21 may be substituted for lens shell 11 in lens assembly 51. In such case, it would, of course, be independently filled with one or more lens fluids similar to those mentioned above as lens fluid 58. The filling thereof would be effected through hole 46, after which plug 47 would be inserted therein. Obviously, if such arrangement were employed, housing 52 would also have to be filled with the same or some other suitable sonic energy transmitting fluid, so as to transfer the operative sonic energy from flat acoustically clear rearward wall 23 to the piezoelectric energy converter elements of the transducer.

Perhaps at this time it would also be noteworthy that although lenses l7 and 27 are disclosed herein as being hemispherical lenses, it is within the contemplated scope of this invention that they may be designed by the artisan to have whatever geometrical configurations that is, sizes, radius of curvatures, thickness, etc. will give whatever focal characteristics required for any given transducer design and operational circumstances.

It is also important, of course, as previously suggested, that the aforesaid sonic energy refracting fluid used as interface lens fluid 59 be such that the ratio of sound velocity therein to sound velocity of the ambient water and, hence, the index of refraction decreases with an increase of temperature and vice versa, so as to thereby automatically efi'ect a compensation for the focal distortions that would otherwise occur as a result of a temperature variation in said environmental medium. For such purpose, as also previously suggested, M-nitrotoluol (C H NO appears to be an optimum selection for interface lens fluid 59.

Referring now to FIG. 5, there is shown, in schematic form, the typical operation of the invention. Disclosed is an ambient environmental fluid F having a sound velocity C within which sonic energy rays 61, 62, and 63 are traveling in such manner as to pass through acoustical window 64 and into interface fluid F having a sound velocity C From interface fluid F said acoustical rays pass through acoustical lens 65 and into lens fluid F having a sound velocity C But because of the shape of acoustical lens 65 and the ratio of sound velocities of C /C said acoustical rays are focused at some radial focal length (jl) on electroacoustical energy converter array 66, the outputs of which are supplied to a sonar receiver 67. Under such circumstances, as will be discussed more fully during the explanation of the operation of the invention presented below, the controlling index of refraction, Q, of such lens system, as far as temperature is concerned, is equal to C /C since fluid F is interposed between ambient environment F and lens fluid F FIG. 6 illustrates a representative system that will enable the acoustical lens of FIG. 4 to be used to an advantage at various water depths, but especially at useful ocean depths. Shown is an electroacoustical transducer 71 containing a temperature compensating lens 72, similar to that indicated as acoustical lens assembly 51 in FIG. 4, the output of which is supplied to an electroacoustical energy converter array 73, the output of which is, in turn, connected to the input of a sonar receiver 74. A readout is calibrated and connected to the output of sonar receiver 74 for suitable display and- /or recording of the signals received therefrom.

Lens 72, as previously indicated, is capable of having the amount and pressure of its interface pressure controlled, so that it may, for instance, be operated at numerous useful ocean depth pressures. For this purpose, interface fluid 76 is supplied to a pressure controller 77 which regulates the pressure of the interface fluid supplied to lens 72, so that it will be continuously equalized with the ambient environmental pressure as a result of said ambient pressure being sensed by a pressure sensor 78. Hence, when acoustical energy 79 emanating through sea water 81 from an underwater target 82 is received by lens 72, the image thereof is not distorted enough due to a change in water temperature to adversely affect the indication thereof by readout 75.

There are obviously numerous ways in which the system of FIG. 6 could be deployed, in order to search for targets. Hence, the way exemplified in said FIG. 6 is by means of a carrier vehicle 83, which, of course, may be a marine vehicle, such as a boat, ship, submarine boat or vehicle, or the like. Thus, it should be understood that said system is disclosed merely as an example, rather than as a limitation.

FIG. 7 graphically displays the change of index of refraction effected upon a liquid lens electroacoustical transducer by a change in ambient temperature, both with and without the lens constituting this invention incorporated therein.

THEORY OF OPERATION The index of refraction at the interface of an ultrasonic lens determines the focal length of the lens. In the case where the ultrasonic lens is a fluid lens having an acoustically clear membrane container operating within water, then the index of refraction, n, thereof is expressed by the equation:

n C /C where C is the velocity of sound in water, and

C, is the velocity of sound in the lens fluid.

It has been determined that the velocity of sound in water increases with an increase in temperature, and it has also been determined that the velocity of sound in most acoustical lens fluids decreases with an increase in temperature. Hence, experience has shown that only a few degrees temperature change of liquid lens electroacoustical transducers submerged in water ordinarily causes the index of refraction of the lens thereof to change enough to change the focal length thereof and thereby defocus the lens. Of cousre, such defocusing, in turn, causes acoustical image resolution to be deleteriously effected to the extent that, for most practical purposes, the fidelity thereof is poor. But, if another fluid is inserted between the transducer lens fluid and the water or other ambient medium as an interface fluid, and if said interface fluid is so selected as to cause it to tract the sound velocity variation with temperature of the lens fluid, the combination thereof becomes independent of temperature and, to a considerable extent, the salinity of the ambient medium, in the event it happens to be sea water, or the like. It has been found that M-chloroanaline (ClC6H4NH2) is an is an optimum selection for the interface fluid. When it is included within interface lens shell 35 in combination with a fluid having appropriate sound velocity-temperature characteristics in acoustical lens shell 11 in the lens assembly 51 of FIG. 4 and said lens assembly 51 is made a part of an electroacoustical transducer, the temperature compensation characteristics which result are depicted by line 91 of FIG. 7. As may readily be seen therefrom, the index of refraction of the total lens changes approximately 0.06 or 3.8 percent from 10 to 50C. This, of course, is a considerable improvement over the adverse index of refraction change that would occur in a transducer not incorporating the subject lens, the latter of which is shown to be about 0.29 or 19 percent by line 92 in FIG. 7. As a matter of fact, said improvement has been theoretically calculated to be about a factor of seven a significant improvement,

ple, fresh water having a given acoustic velocity characteristic to another environmental medium say, salt water having a different acoustic velocity characteristic and vice versa.

MODE OF OPERATION The operation of the invention will now be discussed briefly in conjunction with all of the figures of the drawing. i

Perhaps the best representation of an operational system which could incorporate the subject invention to an advantage is depicted in FIG. 6. Assumingan underwater target 82 has been acquired by lens 72 of electroacoustical transducer 72, the sonic image thereof will pass therethrough and be focused on energy converter array 73 (in the manner similar to that exemplarily portrayed in FIG. 5). Of course, because lens 72 issimilar to that shown in FIG. 4, any temperature variation encountered thereby is compensated within it. Thus, sonar 74 and readout 75 can properly process the image signals impinging on array 73 to thereby effect a visual image of the aforesaid acoustically acquired target. And, in addition, in the event pressure compen' sation is required as well say during great water depth operations the interface fluid within lens 72 may be regulated by the combination of components herewith defined as interface fluid supply 76, pressure controller 77, and pressure sensor 78.

As a result of very little change of the index of refraction with change in the ambient water temperature within this invention, as previously indicated, the fidelity of the acquired image is considerably improved. Hence, any search-echo-ranging or other systems incorporating the subject lens may be. rapidly moved through the water and especially through sea water in any direction without distorting the images of targets acquired thereby for most practical purposes. Hence, the subject lens makes a significant contribution to the advancement of lens used in sonar and other electroacoustical transducers.

Obviously, other embodiments and modifications of the subject invention will readily come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing description and the drawing. It is, therefore, to be understood that this invention is not to be limited thereto and that said modifications and embodiments are intended to be included within the scope of the appended claims.

What is claimed is:

1. An acousticallens, comprising in combination:

a first open-ended cylinder;

a first acoustically clear wall integrally connected to one of the open ends of said first openended cylinder;

an aperture centrally located within and through said first acoustically clear wall;

a second acoustically clear wall, having a convexley curved configuration, integrally connected to said first acoustically clear wall around the periphery of said apertures;

a second cylinder telescopically mounted in a fluid tight manner on and extending from that end of said first cylinder which is connected to said first acoustically clear wall;

a third acoustically clear wall, having a first predetermined thickness near the periphery thereof and a second predetermined thickness that is less than said first predetermined thickness which forms a substantially circular acoustically clear window near the center thereof, integrally connected to the periphery of the extended end of said second cylinder in such manner as to be spatially disposed from said first acoustically clear wall and the larger curved surface of the aforesaid convexley curved second acoustically clear wall, thereby effecting a closed chamber therebetween; and

an interface fluid, having an index of refraction that varies in an inverse proportion with that of water over a predetermined range of temperatures, disposed within said closed chamber.

2. The device of claim 1 whrein said first, second, and third acoustically clear walls consist of a low density polyethylene material.

3. The device of claim 1 wherein said first acoustically clear wall has thickness of the order of 0.070 inches.

4. The device of claim 1 wherein said second acoustically clear wall, having a convexley curved configuration, integrally connected to said first acoustically clear wall around the periphery of said aperture comprises a hemispherical acoustical lens.

5. The device of claim 1, wherein said second acoustically clear wall having a convexley curved configuration, integrally connected to said first acoustically clear wall around the periphery of said aperture comprises a low density polyethylene acoustical lens having a thickness of the order of 0.070 inches and a radius of curvature of such dimension as to refract acoustical energy passing therethrough and focus it at a spot having a predetermined focal length therefrom.

6. The device of claim 1, wherein said third acoustically clear wall, having a first predetermined thickness near the periphery thereof and a second predetermined thickness that is less than said first predetermined thickness which forms a substantially circular acoustically clear window near the center thereof, integrally connected to the periphery of the extended one of said second cylinder in such manner as to be spatially dis posed from said first acoustically clear wall and the larger curved surface of the aforesaid convexley curved second acoustically clear wall, thereby effecting a closed chamber therebetween, comprises an acoustical lens with the window portion thereof having a thickness of the order of 0.025 inches.

7. The device of claim 1, wherein said interface fluid,

having an index of refraction that varies in an inverse 5 proportion with that of water over a predetermined range of temperatures, disposed within said closed chamber comprises CIC H NH 8. The invention of claim 1, further characterized by:

a groove extending around the inner surface periphery of said second cylinder; and

a resilient O-ring located in said groove in such manner as to be urged thereagainst and against the outer surface of said first cylinder, so as to thereby effect a fluid-tight seal therebetween.

9. The invention of claim 1, further characterized by:

a threaded filler hole extending through the longitudinal wall of said second cylinder; and

a threaded plug screwed into the aforesaid filler hole.

10. The invention of claim 1, further characterized by a fourth acoustically clear wall connected to the end of said first cylinder that is opposite said first and second acoustically clear walls in such manner as to effect another closed chamber therebetween within said first cylinder.

11. The invention of claim 10, further characterized by an acoustical energy conducting and refracting fluid disposed within said another closed chamber.

12. The device of claim 11, wherein said acoustical energy conducting fluid disposed within said another closed chamber comprises a fluid having predetermined sound velocity-temperature characteristics.

13. The invention of claim 1, further characterized by:

a first flange connected to the outside diameter of said first cylinder at the end thereof opposite said first and second acoustically clear walls;

a first plurality of bolt holes extending through and around said first flange, the axes of which are parallel to each other and to the longitudinal axis of said first cylinder;

a second flange connected to the outside diameter of said second cylinder at the end thereof opposite said third acoustically clear wall; and

a second plurality of bolt holes of like number as said first plurality of bolt holes extending through and around said second flange, the axes of which are parallel to each other and the longitudinal axis of said second cylinder, and the axes of which are respectively in alignment with the axes of the aforesaid first plurality of bolt holes.

14. The invention of claim 13, further characterized by means respectively extending through said first and second pluralities of bolt holes for the securing of said first and second flanges in a fixed relationship together.

15. The invention of claim 13, further characterized by means respectively extending through said first and second pluralities of bolt holes for securing said first and second flanges in fixed relationship together and to the housing of an electroacoustical transducer. 

1. An acoustical lens, comprising in combinatIon: a first open-ended cylinder; a first acoustically clear wall integrally connected to one of the open ends of said first open-ended cylinder; an aperture centrally located within and through said first acoustically clear wall; a second acoustically clear wall, having a convexley curved configuration, integrally connected to said first acoustically clear wall around the periphery of said apertures; a second cylinder telescopically mounted in a fluid tight manner on and extending from that end of said first cylinder which is connected to said first acoustically clear wall; a third acoustically clear wall, having a first predetermined thickness near the periphery thereof and a second predetermined thickness that is less than said first predetermined thickness which forms a substantially circular acoustically clear window near the center thereof, integrally connected to the periphery of the extended end of said second cylinder in such manner as to be spatially disposed from said first acoustically clear wall and the larger curved surface of the aforesaid convexley curved second acoustically clear wall, thereby effecting a closed chamber therebetween; and an interface fluid, having an index of refraction that varies in an inverse proportion with that of water over a predetermined range of temperatures, disposed within said closed chamber.
 2. The device of claim 1 whrein said first, second, and third acoustically clear walls consist of a low density polyethylene material.
 3. The device of claim 1 wherein said first acoustically clear wall has thickness of the order of 0.070 inches.
 4. The device of claim 1 wherein said second acoustically clear wall, having a convexley curved configuration, integrally connected to said first acoustically clear wall around the periphery of said aperture comprises a hemispherical acoustical lens.
 5. The device of claim 1, wherein said second acoustically clear wall having a convexley curved configuration, integrally connected to said first acoustically clear wall around the periphery of said aperture comprises a low density polyethylene acoustical lens having a thickness of the order of 0.070 inches and a radius of curvature of such dimension as to refract acoustical energy passing therethrough and focus it at a spot having a predetermined focal length therefrom.
 6. The device of claim 1, wherein said third acoustically clear wall, having a first predetermined thickness near the periphery thereof and a second predetermined thickness that is less than said first predetermined thickness which forms a substantially circular acoustically clear window near the center thereof, integrally connected to the periphery of the extended one of said second cylinder in such manner as to be spatially disposed from said first acoustically clear wall and the larger curved surface of the aforesaid convexley curved second acoustically clear wall, thereby effecting a closed chamber therebetween, comprises an acoustical lens with the window portion thereof having a thickness of the order of 0.025 inches.
 7. The device of claim 1, wherein said interface fluid, having an index of refraction that varies in an inverse proportion with that of water over a predetermined range of temperatures, disposed within said closed chamber comprises ClC6H4NH2.
 8. The invention of claim 1, further characterized by: a groove extending around the inner surface periphery of said second cylinder; and a resilient O-ring located in said groove in such manner as to be urged thereagainst and against the outer surface of said first cylinder, so as to thereby effect a fluid-tight seal therebetween.
 9. The invention of claim 1, further characterized by: a threaded filler hole extending through the longitudinal wall of said second cylinder; and a threaded plug screwed into the aforesaid filler hole.
 10. The invention of claim 1, further characterized by a fourth acoustically clear wall connected to the end of said first cylinder that is opposite said first and second acoustically clear walls in such manner as to effect another closed chamber therebetween within said first cylinder.
 11. The invention of claim 10, further characterized by an acoustical energy conducting and refracting fluid disposed within said another closed chamber.
 12. The device of claim 11, wherein said acoustical energy conducting fluid disposed within said another closed chamber comprises a fluid having predetermined sound velocity-temperature characteristics.
 13. The invention of claim 1, further characterized by: a first flange connected to the outside diameter of said first cylinder at the end thereof opposite said first and second acoustically clear walls; a first plurality of bolt holes extending through and around said first flange, the axes of which are parallel to each other and to the longitudinal axis of said first cylinder; a second flange connected to the outside diameter of said second cylinder at the end thereof opposite said third acoustically clear wall; and a second plurality of bolt holes of like number as said first plurality of bolt holes extending through and around said second flange, the axes of which are parallel to each other and the longitudinal axis of said second cylinder, and the axes of which are respectively in alignment with the axes of the aforesaid first plurality of bolt holes.
 14. The invention of claim 13, further characterized by means respectively extending through said first and second pluralities of bolt holes for the securing of said first and second flanges in a fixed relationship together.
 15. The invention of claim 13, further characterized by means respectively extending through said first and second pluralities of bolt holes for securing said first and second flanges in fixed relationship together and to the housing of an electroacoustical transducer. 